FIG. 1 shows a known differential pulley 10. Differential pulleys are used to lift heavy loads because they achieve a high gear ratio. A standard differential pulley 10 consists of two pulley wheels 12, 14 of different radii R, r which are fast with one another to rotate as one about a common axle. The differential pulley 10 is fixed in position so as not to move. A continuous chain 16 is run around both pulley wheels 12, 14 in opposite directions to create two hanging loops 18, 20. A moveable pulley block 22, which is connected to a load 24, is placed in a first hanging loop 18, and the load 24 is raised or lowered by pulling or releasing respectively a length of the second hanging loop 20. When the load 24 is being raised or lowered, the two pulley wheels 12, 14 rotate as one around the common axle and, because the chain 16 is reaved in opposite directions around each wheel 12, 14, the chain 16 winds upon one pulley wheel as it unwinds from the other. A high gear ratio is achieved because the pulley wheels 12, 14 have different radii. It is important that the chain 16 does not slip on the pulley wheels 12, 14, and so the pulley wheels 12, 14 have lugs (not shown) to engage the chain links and thereby prevent slipping.
The known differential pulley 10 achieves a single high power gear ratio which is determined by the radii of the two pulley wheels 12, 14, and it is not possible to achieve multiple gear ratios from a single known differential pulley 10. However, there are applications where more than one gear ratio is desired.
On a sailing boat 30, for example as shown in FIG. 2, when the mainsail 32 is being controlled, it is advantageous to be able to use a high gear ratio when large forces are acting on the mainsail 32, or when fine control of the position of the mainsail 32 is required. It is also advantageous to be able to trim the sail position quickly when maneuvering, and this requires that the pulley system 34 for the mainsheet 36 is configurable to have more than one gear ratio, preferably having at least one high power gear ratio and one low power gear ratio. The known differential pulley would not be useful for such applications. In the known boat shown in FIG. 2 the mainsheet pulley system acts between the boom 38 and a transverse spar 40 across the boat 30. A pulley system 42 is also used on the known boat 30 for the vang 44 or kicking strap to the boom 38.
Two-speed mainsheet block and tackle systems provide mainsail control with two gear ratios. For example, a mainsheet pulley system as shown in FIG. 3 with gear ratios of 4:1 and 16:1 are commercially available. Two-speed continuous mainsheet block and tackle systems, as shown in FIG. 4, having for example gear ratios of 2:1 and 4:1, 3:1 and 6:1, and 4:1 and 8:1 are also commercially available, for example, the Harken system 330 2-speed mainsheet system. However, only two gear ratios are possible with these block and tackle systems, and furthermore, the available gear ratios from a single system are relatively close to one another, thus preventing both very fine tuning and quick trimming of a mainsail with a single mainsheet system.